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net: No more expensive sock_hold()/sock_put() on each tx 

One of the problem with sock memory accounting is it uses
a pair of sock_hold()/sock_put() for each transmitted packet.

This slows down bidirectional flows because the receive path
also needs to take a refcount on socket and might use a different
cpu than transmit path or transmit completion path. So these
two atomic operations also trigger cache line bounces.

We can see this in tx or tx/rx workloads (media gateways for example),
where sock_wfree() can be in top five functions in profiles.

We use this sock_hold()/sock_put() so that sock freeing
is delayed until all tx packets are completed.

As we also update sk_wmem_alloc, we could offset sk_wmem_alloc
by one unit at init time, until sk_free() is called.
Once sk_free() is called, we atomic_dec_and_test(sk_wmem_alloc)
to decrement initial offset and atomicaly check if any packets
are in flight.

skb_set_owner_w() doesnt call sock_hold() anymore

sock_wfree() doesnt call sock_put() anymore, but check if sk_wmem_alloc
reached 0 to perform the final freeing.

Drawback is that a skb->truesize error could lead to unfreeable sockets, or
even worse, prematurely calling __sk_free() on a live socket.

Nice speedups on SMP. tbench for example, going from 2691 MB/s to 2711 MB/s
on my 8 cpu dev machine, even if tbench was not really hitting sk_refcnt
contention point. 5 % speedup on a UDP transmit workload (depends
on number of flows), lowering TX completion cpu usage.

Signed-off-by: Eric Dumazet <eric.dumazet@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
This commit is contained in:
Eric Dumazet 2009-06-11 02:55:43 -07:00 committed by David S. Miller
parent f2333a014c
commit 2b85a34e91
4 changed files with 30 additions and 7 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

6
include/net/sock.h
View File

@ -1217,9 +1217,13 @@ static inline int skb_copy_to_page(struct sock *sk, char __user *from,
static inline void skb_set_owner_w(struct sk_buff *skb, struct sock *sk)
{
sock_hold(sk);
skb->sk = sk;
skb->destructor = sock_wfree;
/*
* We used to take a refcount on sk, but following operation
* is enough to guarantee sk_free() wont free this sock until
* all in-flight packets are completed
*/
atomic_add(skb->truesize, &sk->sk_wmem_alloc);
}

29
net/core/sock.c
View File

@ -1008,7 +1008,7 @@ struct sock *sk_alloc(struct net *net, int family, gfp_t priority,
}
EXPORT_SYMBOL(sk_alloc);
void sk_free(struct sock *sk)
static void __sk_free(struct sock *sk)
{
struct sk_filter *filter;
@ -1031,6 +1031,17 @@ void sk_free(struct sock *sk)
put_net(sock_net(sk));
sk_prot_free(sk->sk_prot_creator, sk);
}
void sk_free(struct sock *sk)
{
/*
* We substract one from sk_wmem_alloc and can know if
* some packets are still in some tx queue.
* If not null, sock_wfree() will call __sk_free(sk) later
*/
if (atomic_dec_and_test(&sk->sk_wmem_alloc))
__sk_free(sk);
}
EXPORT_SYMBOL(sk_free);
/*
@ -1071,7 +1082,10 @@ struct sock *sk_clone(const struct sock *sk, const gfp_t priority)
newsk->sk_backlog.head = newsk->sk_backlog.tail = NULL;
atomic_set(&newsk->sk_rmem_alloc, 0);
atomic_set(&newsk->sk_wmem_alloc, 0);
/*
* sk_wmem_alloc set to one (see sk_free() and sock_wfree())
*/
atomic_set(&newsk->sk_wmem_alloc, 1);
atomic_set(&newsk->sk_omem_alloc, 0);
skb_queue_head_init(&newsk->sk_receive_queue);
skb_queue_head_init(&newsk->sk_write_queue);
@ -1175,12 +1189,18 @@ void __init sk_init(void)
void sock_wfree(struct sk_buff *skb)
{
struct sock *sk = skb->sk;
int res;
/* In case it might be waiting for more memory. */
atomic_sub(skb->truesize, &sk->sk_wmem_alloc);
res = atomic_sub_return(skb->truesize, &sk->sk_wmem_alloc);
if (!sock_flag(sk, SOCK_USE_WRITE_QUEUE))
sk->sk_write_space(sk);
sock_put(sk);
/*
* if sk_wmem_alloc reached 0, we are last user and should
* free this sock, as sk_free() call could not do it.
*/
if (res == 0)
__sk_free(sk);
}
EXPORT_SYMBOL(sock_wfree);
@ -1819,6 +1839,7 @@ void sock_init_data(struct socket *sock, struct sock *sk)
sk->sk_stamp = ktime_set(-1L, 0);
atomic_set(&sk->sk_refcnt, 1);
atomic_set(&sk->sk_wmem_alloc, 1);
atomic_set(&sk->sk_drops, 0);
}
EXPORT_SYMBOL(sock_init_data);

1
net/ipv4/ip_output.c
View File

@ -498,7 +498,6 @@ int ip_fragment(struct sk_buff *skb, int (*output)(struct sk_buff *))
BUG_ON(frag->sk);
if (skb->sk) {
sock_hold(skb->sk);
frag->sk = skb->sk;
frag->destructor = sock_wfree;
truesizes += frag->truesize;

1
net/ipv6/ip6_output.c
View File

@ -680,7 +680,6 @@ static int ip6_fragment(struct sk_buff *skb, int (*output)(struct sk_buff *))
BUG_ON(frag->sk);
if (skb->sk) {
sock_hold(skb->sk);
frag->sk = skb->sk;
frag->destructor = sock_wfree;
truesizes += frag->truesize;